The determination of gas and liquid flow rates in gas-liquid fluid mixtures is important in the oil and gas industry.
An example of an apparatus for measuring such flow rates is Schlumberger's Vx™ system (see e.g. I. Atkinson, M. Berard, B.-V. Hanssen, G. Ségéral, 17th International North Sea Flow Measurement Workshop, Oslo, Norway 25-28 Oct. 1999 “New Generation Multiphase Flowmeters from Schlumberger and Framo Engineering AS”) which comprises a vertically mounted Venturi flow meter, a dual energy gamma-ray hold up measuring device and associated processors. This system allows the simultaneous calculation of gas, water and oil volumetric flow rates in multi phase flows.
However, with conventional implementations of Vx™ technology the accuracy of the calculations starts to degrade as the gas volume fraction (GVF) increases above about 90%. In particular, at high GVF it can be difficult to determine properties of the liquid phase.
WO 2004/106861 proposes a multi phase flowmeter in which a twisted tape device is used to separate a liquid phase from a liquid-gas mixture into an annular film on the tube wall of a straight pipe section. Measurements are made to determine the liquid flow rate in the film. Following the straight pipe section, the liquid is re-entrained into the gas stream by an expansion contraction system. The homogenised flow is then passed to a Venturi.
GB A 2447490 proposes an apparatus for investigating the properties of a gas-liquid fluid mixture. The apparatus includes a conduit within which the fluid mixture is induced to swirl, for example, by a helical insert or vane assembly within the conduit, or a tangential flow inlet to the conduit. The swirling flow forces the liquid and gas to separate, the liquid forming a surface layer on the wall of the conduit. The separated swirling flow is then introduced into a Venturi, the liquid undergoing centrifugal acceleration in the throat of the Venturi. The apparatus may have a further upstream Venturi through which the mixture flows without swirling. Measurements, such as pressure differences, liquid layer velocities and liquid hold-ups, taken at suitable points along the flow allow, for example the gas and the liquid mass flow rates to be determined.
Thus swirling flows can be applied to develop apparatuses which allow useful measurements to be made on gas-liquid mixtures.
However, conventional devices for inducing swirling flow have disadvantages. Conventional tangential inlet devices tend to produce large pressure losses as the flow is directed around several 90° doglegs. They also have large footprints which can make their deployment impractical in the limited space of e.g. an offshore platform. Helical insert or vane assemblies produce lower pressure losses, but are less effective at swirling the flow.